This invention relates generally to means for monitoring temperatures of semiconductors, and more particularly to the monitoring of junction temperatures of power semiconductors subject to high currents.
The use of solid state semiconductor devices such as diodes and thyristors is now well known in electric power applications. It is also well known that much of the heat generated in a power semiconductor originates at its current carrying junction or junctions. As a result, the junction attains the highest temperature in the semiconductor and is the point at which semiconductor failure usually occurs, should the temperature become excessive. The term "junction" is used in this specification in its broadest sense. Semiconductors often vary in the number of individual junctions that carry current from anode to cathode. For example, a diode may have a single junction while a thyristor will normally have a number of junctions. In this specification, including the claims, the use of the term "junction" is intended to embrace both single and multiple junction semiconductors.
Junction temperature determination cannot be accomplished with a satisfactory degree of accuracy by measuring the temperature of the structure employed to remove heat from the semiconductor element. Owing to the slow thermal transfer characteristics of these structures, external temperature measurements do not accurately reflect junction temperature. In applications of power semiconductors, it is impractical to measure junction temperature directly; as a result, the protective devices used have taken an indirect approach. The oldest and best known of such protective devices is, of course, the fuse, the particular fuse used being designed to blow at a current value considered safe for the semiconductor. This is a somewhat "rough and ready" approach, an approach which tends not to utilize the full current carrying capabilities of the semiconductor and one which is not normally fast enough to protect the semiconductor against all transient conditions. Moreover, fuses introduce the problem of matching the thermal characteristics of the fuse to the semiconductor being used.
Solid state electronic circuits are also known for protecting power semiconductors against overtemperatures. As would be expected, these circuits can be made more sensitive and faster acting than fuses. A circuit of this nature is disclosed in U.S. Pat. No. 3,622,849, "Thyristor Junction Temperature Monitor", issued Nov. 23, 1971, to F. W. Kelley and F. L. Steen. This patent discloses a circuit which combines a signal representing the temperature of the heat dissipating structure of the semiconductor with a signal representing the current flowing in the semiconductor to arrive at a third signal representing the temperature of the junction of the semiconductor. It is summarized in the patent in the following words:
A temperature monitor is provided for determining if the junction temperature of a conducting thyristor has reached a preselected level corresponding to a dangerously elevated junction temperature. The monitor comprises circuitry which synthesizes the thermal response of a portion of the thyristor and the associated pressure assembly between the interior PN-junction of the thyristor and an external reference point on the heat sink closely adjacent the junction. The synthesizing circuitry utilizes a measured signal indicative of the current level flowing through the junction and converts this signal to a signal indicative of the power dissipated in the junction. The latter signal is supplied to a thermal transfer simulator where it is converted into a signal indicative of the temperature difference between the junction and the heat sink reference point. This signal is combined in an adder circuit with a measured signal indicative of the heat sink temperature to provide an output signal indicative of the junction temperature per se. The output of the adder circuit is preferably supplied to a lever detecter for providing a stop signal when its input is above a preselected level. Means responsive to the stop signal may then be used to suppress the gate signals to the conducting thyristor so that it turns off and cools down.